Power of a lens is defined as the ability of lens to converge or to diverge a beam of light. It is also defined as reciprocal of focal length in metres.
For lens A :
${\mathit{f}}_{\mathrm{A}}=+10\mathrm{cm}=0.1\mathrm{m}$
Since the focal length is positive, it is a convex lens.
Now, ${\mathrm{P}}_{\mathrm{A}}=\frac{1}{{\mathit{f}}_{\mathrm{A}}}=\frac{1}{0.1}=+10\mathrm{D}$.
For lens B :
${\mathit{f}}_{\mathrm{B}}=-10\mathrm{cm}=-0.1\mathrm{m}$
Since the focal length is negative, it is a concave lens.
${\mathrm{P}}_{\mathrm{B}}=\frac{1}{{\mathit{f}}_{\mathrm{B}}}=\frac{1}{-0.1}=-10\mathrm{D}$
In case of a convex lens, image is virtual and magnified if object is placed between focus and optical centre.